Physicians commonly treat blood vessel occlusions by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, physically holding open and, if desired, expanding the wall of affected vessels. Typically, stents can compress for insertion through small lumens via catheters and then expand to a larger diameter once they are positioned. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Physicians also use stents for providing biological therapy by medicating the stents. Medicated stents allow local administration of a drug. This is preferred because these stents concentrate the drug at a specific site and thus deliver smaller total medication levels in comparison to systemic dosages.
One stent medicating method involves using a coating of a polymeric carrier on the stent. Coating comprises immersing the stent in, rolling the material on, applying the material to, or spraying the stent with a material including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the solvent. Then the solvent evaporates, leaving a polymer and drug coating. After stent implantation, the stent releases the drug in a sustained manner.
U.S. Pat. No. 6,139,573, Sogard et al. teaches an elongated, radially expandable, tubular stent and a polymeric layer covering and conforming to its surface. It teaches laminating a polymeric liner layer and an external polymer layer together to form a composite structure, containing the stent, and at least three domains of distinct porosity. It teaches making the stent from a variety of materials including stainless steel, titanium, platinum, gold, or other biocompatible metals. Furthermore, it teaches that the polymeric layers are expanded polytetrafluoroethylene (ePTFE).
In U.S. Pat. No. 6,010,530, Goicoechea teaches a self-expanding stent encapsulated by a skin. The stent contains a continuous, zigzag, nitinol wire wound into several concentric hoops. Its skin is an elastomeric polymer, such as Chronoflex (available from Poly-Medica Biomaterials Inc., Woburn, Mass.).
In U.S. Pat. No. 5,749,880, Banas et al. teach an encapsulated stent that comprises at least one stent member concentrically interdisposed between at least two tubular ePTFE extrudates, each of the extrudates have a uniaxial fibril microstructure oriented parallel to the longitudinal axis of the stent.
United States Patent Application Publication No. US 2002/0133224 A1 discloses a stent encapsulated with a microporous polymeric membrane. An electrostatic deposition process provides stent encapsulation.
Current stents have an overall cylindrical shape with a complex pattern of struts. When placed in the target vessel and expanded, the stent occupies about 10-25% of the vessel wall surface area. Stents are an unusual medical device in that their design is a compromise between mechanical function and biological impact. Skilled Artisans want stents to mechanically support the vessel. This argues for high wall coverage to give good scaffolding, stopping all plague prolapse. But since stents can cause biological responses that precipitate in-stent restenosis, or thrombosis skilled artisans want biologically invisible stents, which tends toward low wall coverage.
In addition to non-covered stents, covered stents are also known. Physicians use these devices for certain niche applications. These often serve as bailout devices in cases of severe dissection or perforation of the arterial wall. They are also used to treat aneurysms that may form in the vessel wall from disease or trauma. Their mechanical limitations center on their deliverability and larger profile compared to regular stents. Biological challenges include not only restenosis, but also a higher incidence of thrombotic complications due to the larger surface area of synthetic material. But covered stents could deliver a higher drug payload, and deliver this drug more uniformly to the vessel wall. Between the arterial wall coverage of bare metal stents and that of fully covered stents, lies a continuum in the extent of vessel wall coverage. The best coverage is the minimum amount needed to accomplish the mechanical task without creating adverse biological responses.
There is a place for covered stents in interventional cardiology but they still have the following issues:                The covering increases the stent profile because it lies on the stent's outer surfaces so it is external to the stent;        The covering must expand with the stent; tearing is possible if the covering is not sufficiently elastic;        Attachment of the covering to the stent is problematic; insecure attachment can lead to the covering becoming loose or folding over;        A covered stent that is too impermeable can completely isolate the underlying endothelium/smooth muscle from its blood supply, which can cause tissue death or necrosis.        